Anaerobic digestion has many advantages both environmentally and economically. First, it produces renewable energy in the form of methane, a renewable energy source, which leads to a steady increase in the number of anaerobic manure digesters in the United States. According to the report from the World Dairy Expo held in Madison, Wisconsin in 2009 (Expo'09, 2009), the U.S. dairy industry is taking the lead in adopting anaerobic technology because the majority (over 75%) of operating US manure digesters is installed on dairy farms. It is anticipated that this trend will continue as the country has determined to reduce its reliance on ever diminishing fossil-based energy resources. Second, the technology can significantly reduce the polluting strength of the treated waste materials, such as chemical oxygen demand (COD), thus ameliorating their pollution potential to the environment when discharged. Due to the nature of dairy operations, a tangible amount of milk coming from the milking parlor wastewater is often discharged to the bulk manure, which can dramatically increase the COD level of such waste streams. The high COD content (190,000 mg/L) of milk makes the common practice of land applying the milk contaminated manure dangerous due possibly to the potential of causing severe contamination of surface and ground waters from runoff and leaching. Such practice is therefore drawing increased scrutiny from the public and environmental regulatory agencies. Fortunately, with the number of dairy producers willing to adopt anaerobic digesters on their farms continuing to grow, the concern for such pollution could be tempered. However, a remaining question of this remedy is whether the added milk has any impact on the overall digestion process in terms of biogas production and pollutants removal.

What we did

In this project, the overall response of co-digesting dairy manure with milk added at 7 different levels, i.e., 1, 3, 5, 7, 9, 14, and 19%, using lab-scale batch anaerobic digesters was investigated. The co-digestion performance was evaluated based on total biogas volume production, methane concentration, and its volume in the biogas generated. The changes and/or reductions in COD of the treated liquid were also presented.

What we found

Cumulative biogas production affected by different milk content

Figure 1 presents the means of cumulative biogas volume produced for all the milk concentrations tested over the entire experimental period. With reference to the control, there are a number of observations that appear to be uniquely caused by milk additions. First, it was clearly shown that increasing milk content could increase the cumulative biogas production during the operation, with the total volume of biogas produced being 5260, 5790, 6300, 7010, 7480, 8960, and 10150 mL for the milk treatments of 1, 3, 5, 7, 9, 14, and 19%, respectively, as opposed to the control (4980 mL). Second, higher milk content could significantly raise the initial biogas production rate, as indicated by the slopes of all the curves at the beginning of the experiment, which started to climb steeply after only 6 hours into the digestion process. Third, the presence of milk appeared to have some influence on the stability of the digestion process, as evidenced by the fluctuation of biogas production curves at high milk concentrations. For instance, the treatments having milk content up to 7% demonstrated a similar trend. But for milk content of 9, 14, and 19%, the fluctuation in biogas production volume became progressively conspicuous. Especially for the 19% milk treatment, the biogas volume produced first jumped from 190 mL at 6 hour to 1190 mL at 12 hour after the digestion started, followed by a relatively moderate production period before it jumped again after 8 days of digestion. Considering the results from this study, it may be concluded that milk can increase biogas production when co-digested with dairy manure.

Figure 1 also indicated that the biogas production rates (slopes of the curves) were in general similar for the treatments up to milk content of 9% but for the last two milk concentrations (14 and 19%), the production rates increased sharply at the beginning and then dropped slightly before catching up with other treatments after day 5 (for 14%) and day 10 (for 19%). The maximum production rates for all treatments but 19% could be determined by obtaining the slopes in the first ten days, which were 13.9, 14.9, 16.4, 18.1, 20.2, 21.0, and 22.3 mL/h for the control, 1, 3, 5, 7, 9, and 14% milk content, respectively. For the 19% milk content, the maximum rate (excluding the sudden increases at the beginning of the experiment) actually occurred between day 10 and 15, which was 23.2 mL/h. It is interesting to note that when most biogas production rates began to level off after day 10 or 15 (14% only), the 19% milk treatment continued to show a higher production rate all the way to the end of experiment, indicating that the biogas production could potentially last much longer because of the sufficient substrates provided by the added milk. Again, the data have verified that adding milk in dairy manure could increase volumetric biogas production. Increases in biogas generation rate by 7 and 18% were obtained when milk was added to the manure digesters at merely 1 and 3% (v/v), respectively.

Cumulative CH4 volumes affected by different milk content

Figure 2 shows the performance of different treatments in cumulative CH4 production, which indicates that adding milk to dairy manure digestion will promote the volumetric production of both biogas and methane. However, the CH4 content in the produced biogas deteriorated as the milk content increased (from 66.5% for the control to 63.5% for 19% milk treatment, Figure 3). It can thus be inferred that although the volumes of total biogas and methane were increased by increasing the milk content in the digester, the increase in methane volume was not in tandem with that in total biogas volume (as shown in Figure 3 that the two regression lines had different slopes), implying that a significant amount of CO2 was concurrently produced. Apparently, the presence of milk in the digestion substrate is the only legitimate cause for the increasing production of CO2. Also from Figure 3, although the effect of milk on lowering the CH4 content in biogas is observed for all milk treatments, the extent of such an effect is different. The milk impact on CH4 content in biogas was not significant for manure containing milk up to 3% (v/v), but it turned significant at 5%. Summarizing the above discussions leads to an intuitive suggestion that in order to avoid production of a substantial amount of CO2 due to the spilled milk in the digestion process, dairy producers should manage to control the milk content in the digester liquid ≤3%.

COD, TKN, and C/N ratio changes in digestates from the digestion of dairy manure with milk

Table 1 shows the average values of COD, TKN, and C/N ratios before and after digestion for all the treatments, including the percent reductions of COD and TKN. The added milk substantially increased the digester content COD as the amount of milk increased. However, at the end of the experiment, the final COD concentration in most digester effluent samples reached a fairly similar level, suggesting that the digestion process for the majority of the treatments was completed properly. In addition, since all the experiments were run on the same time schedule, the COD degradation efficiency obviously increased with increasing milk addition from 49.7% for the control to 77.8% for the 19% milk treatment. The improved COD removal efficiency in company with the increasing milk content could be attributed to the gradually elevated C/N ratio due to the added milk (from 5.19 for the control to 10.7 for the 19% treatment) because it is recognized that the optimum C/N ratio for anaerobic digestion is around 20/1 to 30/1, which could explain the continuous increase in COD removal as the C/N ratio increased as shown in Table 1. At the end of experiment, the effluent C/N ratio averaged 2.75, which was very close to the value for the digested dairy manure (2.83). As for TKN, the removal efficiency is almost negligible, which is the typical behavior commonly observed for anaerobic digestion, indicating that the digestion operation was carried out successfully. Based on the information obtained from this study, it may be concluded that milk content up to 19% (v/v) in dairy manure may have little negative impact on the COD removal efficiency in the anaerobic digestion process.

Table 1. The average values of COD and TKN for the batch digestion process with different milk content.

Conclusion

The data from this study indicated that milk could increase biogas volumetric production if co-digested with dairy manure. The added milk also increased the maximum biogas production rate as compared to the control (7 and 18% increases were observed when milk was added to the digested manure at merely 1 and 3% levels, respectively). However, it was observed that milk addition could reduce the methane content in the biogas produced, implying a potential increase in CO2 production. To avoid that, the milk content in the digester liquid should be limited to below 3%. The COD degradation efficiency was not affected by the added milk, and rather, an increase in efficiency was observed with increasing milk content from 49.7% for the control to 77.8% for the digester with 19% milk treatment. The information from this study could largely relieve the concern of dairy producers who employ anaerobic digesters to treat manure together with the milk containing wastewater from milking parlor operations in terms of the potential impact on the digestion process and the environment.